Ultraviolet-curable adhesive for bonding optical disks

ABSTRACT

An ultraviolet-curable adhesive having an excellent moisture resistance and suitable for bonding optical disks having an information recording layer (vapor deposition layer), particularly optical disks having a thick vapor deposition layer, which contains, as essential components, (A) N-vinylpyrrolidone, (B) a urethane (meth)acrylate oligomer derived from a caprolactone-based polyol, an alicyclic diisocyanate and (meth)acrylic acid, and (C) a photopolymerization initiator containing (C-1) a photopolymerization initiator composed of only a carbon atom, a hydrogen atom and an oxygen atom without containing a nitrogen atom, a sulfur atom and a phosphorus atom and (C-2) a photopolymerization initiator containing a nitrogen atom and/or a sulfur atom in a (C-1)/(C-2) ratio of 70/30 to 100/0 by weight.

TECHNICAL FIELD

The present invention relates to an ultraviolet-curable adhesive forbonding optical disks, and more particularly to an ultraviolet-curableadhesive composition which is used in producing information recordingmedia by applying it to an information recording layer of an opticaldisk having the information recording layer on one surface and bondingthereon another optical disk which may have an information recordinglayer.

BACKGROUND ART

Production of optical disks (hereinafter referred also to as “DVD”) usedas an information recording medium has been made in such a manner asapplying an adhesive to an information recording layer of an opticaldisk having the information recording layer on one surface andlaminating onto the information recording layer another optical diskwhich may have an information recording layer and curing the adhesive.

Ultraviolet-curable adhesives are suitable for use in this method fromthe viewpoints of simplification of DVD production steps, no occurrenceof warping of disks owing to heating of the disks. In that case, it isrequired for curing the adhesive that ultraviolet rays irradiated fromthe outside to disks having an information recording layer and aprotective layer and laminated with the adhesive pass through thesubstrate, the information recording layer, the protective layer and thelike. For this reason, it was recognized difficult to make ultravioletrays reach the adhesive to cure it. Thus, it was popular to use hot meltadhesives or thermosetting adhesives.

However, hot melt adhesives have the problem that the adhesives are easyto melt again if heat is applied to DVD using them, so the mechanicalstrength cannot be retained at high temperatures. Also, in case ofproducing DVD by using thermosetting adhesives, problems such aswarping, deformation and the like owing to heating arise.

Thereafter, as a result of investigation in every direction about theuse of ultraviolet-curable adhesive, there are proposedultraviolet-curable adhesives which can be cured even by a slight amountof ultraviolet rays reached passing through the substrate, informationrecording layer, protective layer and the like. For example, known arean ultraviolet-curable adhesive of radical polymerization type whichcontains a (meth)acrylic monomer or oligomer as a main component and aphotopolymerization initiator such as a benzoyl alkyl ether compound, abenzophenone compound or a thioxanthone compound, an ultraviolet-curableadhesive of polyene-polythiol addition polymerization curing type, andan ultraviolet-curable adhesive of cationic polymerization typecomprising an epoxy resin and a Lewis acid type photopolymerizationinitiator (Japanese Patent Publication Kokai No. 8-161771).

However, since a thin film of a metal such as aluminum is used in theinformation recording layer and is easy to be attacked by a cationicpolymerization type adhesive, ultraviolet-curable (meth)acrylicadhesives of radical polymerization type are popularly used at present.

Most of photopolymerization initiators to be used in theultraviolet-curable (meth)acrylic adhesives have an absorption also in ahigh wavelength region. In general, initiators having an absorption at awavelength of 360 nm or more and whose molar extinction coefficient inthat wavelength region is as large as possible are used (Japanese PatentPublication Kokai No. 9-31416, No. 9-35335, No. 9-169956 and No.10-8018).

However, if an optical disk wherein the adhesion is performed by theultraviolet-curable (meth)acrylic adhesive of radical polymerizationtype is allowed to stand in a high humidity atmosphere, there arises aproblem that moisture is easy to be adsorbed by the adhesive layer andthe adsorbed moisture stays at the interface between the adhesive layerand the disk, and also a component in the adhesive migrates, so theseadversely affect the adjacent information recording layer to remarkablylower the recording performance. A translucent film (vapor depositionfilm) of Al, Au, SiN, SiC, Si or the like is generally used as theinformation recording layer. The thickness of the film is usually from50 to 300 angstroms. In case of aluminum deposition film, the thicknessis from 400 to 1,000 angstroms. If the translucent films become thick,the bad influence of moisture on the recording performance of thesetranslucent films particularly increase. Also, since a component itselfof the photopolymerization initiator may corrode the metal thin filmdepending on the kind of the initiator, the bad influence on theinformation recording layer further increases in that case.

An object of the present invention is to provide an ultraviolet-curableadhesive which is hard to cause staying of moisture in the interfacebetween the adhesive layer and the information recording layer even inthe case that a relatively thick translucent film is used, thus scarcelyexerting a bad influence on the information recording layer, and whichdoes not cause separation of the laminated disks.

DISCLOSURE OF INVENTION

In general, an adhesive using a nitrogen atom-containing monofunctionalmonomer such as N-vinylpyrrolidone has a high water absorption and iseasy to introduce a problem in various uses. For such a reason, ingeneral N-vinylpyrrolidone has not been used for applications requiringa moisture resistance.

The present inventor has found that an ultraviolet-curable compositioncontaining N-vinylpyrrolidone which is a nitrogen atom-containingmonofunctional monomer, and a specific urethane (meth)acrylate oligomeras essential components has, contrary to expectations, a good moistureresistance and is useful as an adhesive for bonding optical disks whichrequires a moisture resistance, and exhibits excellent characteristicsas compared with conventional adhesives.

Thus, in accordance with the present invention, there is provided anultraviolet-curable adhesive for bonding optical disks, which ischaracterized in containing:

(A) N-vinylpyrrolidone,

(B) a urethane (meth)acrylate oligomer derived from a caprolactone-basedpolyol, an alicyclic diisocyanate and (meth)acrylic acid, and

(C) a photopolymerization initiator containing (C-1) aphotopolymerization initiator composed of only a carbon atom, a hydrogenatom and an oxygen atom without containing a nitrogen atom, a sulfuratom and a phosphorus atom and (C-2) a photopolymerization initiatorcontaining a nitrogen atom and/or a sulfur atom in a (C-1)/(C-2) ratioof 70/30 to 100/0 by weight.

The photopolymerization initiator (C) may be composed of only thecomponent (C-1) or may be a combination of the components (C-1) and(C-2).

The adhesive of the present invention may further contain (D) 5 to 100parts by weight of an alicyclic mono(meth)acrylate and/or (E) 5 to 95parts by weight of a bisphenol type di(meth)acrylate per 100 parts byweight of the total of the components (A) and (B), provided that theamount of the component (E) does not exceed the amount of the component(B).

The adhesive of the present invention satisfies such characteristicsrequired for adhesives for bonding optical disks that it has an adequatesoftness, structural destruction of a cured resin does not occur even ifsubjected to a moisture resistance test, the adhesion property to diskmaterials is not lowered, and the performances of a metal film are notlowered. Also, since the adhesive contains N-vinylpyrrolidone and aspecific urethane (meth)acrylate oligomer as the constituent components,the adhesive layer has a high water retainability and accordingly ishard to cause problems such as staying of the absorbed moisture in theinterface between the information recording layer and the adhesive layerand deterioration of the adhesion property, thus exhibiting an excellentmoisture resistance.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a plan view of a specimen wherein disk 1 and disk 2 are bondedwith an adhesive and which is used when evaluating the water resistanceof an ultraviolet-curable adhesive and the corrosion resistance of theinformation recording layer.

BEST MODE FOR CARRYING OUT THE INVENTION

The ultraviolet (UV)-curable adhesive of the present invention suitablefor use in bonding optical disks contains N-vinylpyrrolidone (A),urethane (meth)acrylate oligomer (B) derived from a caprolactone-basedpolyol, an alicyclic diisocyanate and (meth)acrylic acid, and aphotopolymerization initiator (C) for the components (A) and (B). TheN-vinylpyrrolidone (A) provides a water retainability to the curedadhesive layer. Since the moisture absorbed in the adhesive layer isstably trapped by the adhesive layer and accordingly is hard to stay inthe interface between the adhesive layer and the information recordinglayer, the N-vinylpyrrolidone (A) serves to reduce a bad influence ofmoisture on the adjacent information recording layer.

The N-vinylpyrrolidone (A) is polymerizable by light irradiation andserves to raise the reactivity of the polymerization system containingthe component (B), since it has one highly reactive C═C bond, inaddition to having particularly good hydrophilic property and waterretaining property among nitrogen atom-containing monofunctionalmonomers and also having a high adhesion property. Also, since it is amonofunctional monomer and has a low molecular weight, the viscosity ofthe obtained adhesive can be lowered.

As mentioned above, the urethane (meth)acrylate oligomer (B) is acompound derived from a caprolactone-based polyol, an alicyclicdiisocyanate and (meth)acrylic acid, and provides a softness to theadhesive layer.

Preferably, the molecular weight of the urethane (meth)acrylate oligomer(B) is from 700 to 5,500, especially from 1,000 to 5,000, from theviewpoints of mechanical properties, polymerizability, viscosity and thelike. If the molecular weight is too small, the moisture resistance ofthe adhesive tends to lower or the mechanical properties of the adhesivelayer tend to lower. If the molecular weight is too large, there is atendency that the viscosity of the adhesive raises, crystallizationoccurs or the polymerizability lowers.

As the caprolactone-based polyol of the raw materials of the oligomer(B) are used those having a molecular weight of 250 to 4,000 and afunctionality of 1 to 4. Examples of the polyol are, for instance,reaction products of a caprolactone with an alcohol having an aliphatic,aromatic or heterocyclic group such as butanol, ethylene glycol,1,4-butanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane,diethylene glycol, 1,4-dimethylolcyclohexane, bisphenol A polyethoxydiolhaving a molecular weight of 300 to 1,500 or polytetramethylene glycolhaving a molecular weight of 500 to 1,500. These may be used alone or inadmixture thereof. Of these, caprolactone-based diols or triols having amolecular weight of 250 to 3,000 are preferred from the viewpoints ofgood balance of moisture resistance, heat resistance and softness andeasiness in synthesis. Examples of the caprolactone-based diols are, forinstance, e-caprolactone-based diols of the formula:

wherein p and q are independently from 1 to 5, and R₁ is a bivalentgroup that hydroxyl groups are removed from dihydric alcohols among theabove-mentioned alcohols. Examples of the caprolactone-based triol are,for instance, e-caprolactone-based triols of the formula:

wherein x, y and z are independently from 1 to 5, and R₂ is a trivalentgroup that hydroxyl groups are removed from trihydric alcohols among theabove-mentioned alcohols. Examples of the commercially availablecaprolactone-based polyol are, for instance, Placcel 205, Placcel 220,Placcel 240, Placcel 303, Placcel 308, Placcel 405, Placcel 410 (whichare products of DAICEL CHEMICAL INDUSTRIES, LTD.), TONE® Polyol 0210,TONE® Polyol 0240, TONE® Polyol 0301, TONE® Polyol 0310 (which areproducts of UNION CARBIDE JAPAN K.K.), and the like.

Examples of the alicyclic diisocyanate used as a raw material for thecomponent (B) are, for instance, 4,4′-dicyclohexylmethane diisocyanate,isophorone diisocyanate, dicyclopentanyl diisocyanate, and the like.Since the alicyclic diisocyanate is used, the obtained cured product ofthe adhesive has a good weatherability. Of these, isophoronediisocyanate is preferably used, since the balance between the moistureresistance and the weatherability is particularly good.

Further, (meth)acrylic acid is used as a raw material for the component(B). It imparts a photopolymerizability to the component (B).

Since the polyol used as a raw material of the component (B) is acaprolactone-based polyol, oligomers excellent in moisture resistance,weatherability and the like are obtained as compared with other polyols,e.g., polyether polyols and polyester polyols excepting thecaprolactone-based polyols. Also, since a carprolactone-based polyol iscombined with the caprolactone-based polyo or (meth)acrylic acid throughan alicyclic diisocyanate, the weatherability and heat resistance arefurther increased as compared with the use of aromatic diisocyanates andlinear aliphatic diisocyanates.

Commercially available urethane (meth)acrylate polymers can be used asthe component (B). Preferable examples thereof are, for instance,KAYARAD UX-4101 (made by NIPPON KAYAKU CO., LTD.), U-180A (made bySHIN-NAKAMURA CHEMICAL CO., LTD.), KMR 7595, KMR 7610 and KMR 7619(which are products of DAICEL CHEMICAL INDUSTRIES, LTD.), and the like.These may be used alone or in admixture thereof. Of these, KMR 7595, KMR7610 and KMR 7619 are preferred from the viewpoints of good moistureresistance and good balance between the moisture resistance and the heatresistance.

In addition to the components (A) and (B), the photopolymerizationinitiator (C) for polymerizing them are incorporated into the adhesiveof the present invention. As the photopolymerization initiator (C), aphotopolymerization initiator (C-1) composed of only a carbon atom, ahydrogen atom and an oxygen atom without containing a nitrogen atom, asulfur atom and a phosphorus atom is used alone or in combination with aphotopolymerization initiator (C-2) containing a nitrogen atom and/or asulfur atom.

The photopolymerization initiator (C-1) has the advantages that failurecaused by moisture, e.g., corrosion of the information recording layer,is hard to occur in the moisture resistance test, a high adhesionproperty is obtained, balance between the surface curability of theadhesive layer and the deep curability (curability of the adhesive layerin the vicinity of the interface between the adhesive layer and asubstrate located on the side opposite to the ultraviolet irradiationside with respect to the adhesive layer) is good, and a stable adhesivewhich does not start the polymerization even if allowed to stand underirradiation of a fluorescent light.

Examples of the photopolymerization initiator (C-1) are, for instance,1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenylpropanone,2,2-dimethoxy-2-phenylacetophenone, benzophenone, benzoin isobutylether, benzoin n-butyl ether, 2,2-dimethoxy-1,2-diphenylethane-1-one,methylphenyl glyoxylate, benzyl dimethyl ketal, and the like. These maybe used alone or in admixture thereof. Of these, 1-hydroxycyclohexylphenyl ketone and 2-hydroxy-2-methyl-1-phenylpropanone are preferredfrom the viewpoints of good deep curability and adhesion property.

On the other hand, the photopolymerization initiator (C-2) containingnitrogen atom and/or sulfur atom exhibits a good curability with arelatively small amount of transmitted light. Preferred are those whichhave a large absorption on the high wavelength side of not less than 360nm and that an overlap of the wavelength of transmitted light and theabsorption wavelength is large, since a particularly excellentcurability is exhibited.

Examples of the photopolymerization initiator (C-2) are, for instance,Michler's ketone, 2,4-diethylthioxanthone, 2,4-diisopropyl-thioxanthone,2-isopropylthioxanthone, 2-chlorothioxanthone,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,3,6-bis(2-methyl-2-morpholinopropanonyl)-9-butyl-carbazole,isoacryl-4-(dimethylamino)benzoate, ethyl-4-(dimethylamino)benzoate,[4-(4-methylphenylthio)phenyl]phenylmethanone,2-methyl-1-[4-(methyl-thio)phenyl]-2-morpholino-propane-1-one, and thelike. These may be used alone or in admixture thereof. Of these,2-methyl-1-[4-(methyl-thio)phenyl]-2-morpholino-propane-1-one isparticularly preferable from the viewpoint of moisture resistance andcorrosion resistance.

In the adhesive of the present invention, the component (C-1) may beincorporated alone or may be incorporated together with the component(C-2). In case of the combination use of the components (C-1) and (C-2),it is possible to obtain a more excellent adhesive having both anexcellent moisture resistance produced by the component (C-1) and anexcellent curability under a slight transmitted light produced by thecomponent (C-2). From the viewpoint of capable of sufficientlyexhibiting the effects of both initiators, it is preferable that the(C-1)/(C-2) ratio is from 99/1 to 70/30 by weight, especially from 98/2to 90/10 by weight.

With respect to the contents of the respective components included inthe adhesive of the present invention comprising the components (A) to(C), preferably the amount of the component (A) is from 5 to 60% byweight, especially from 10 to 50% by weight, more especially from 20 to50% by weight, based on the total of the components (A) and (B). If theamount of the component (A) is less than 5% by weight, the moistureresistance which is a primary object of the present invention is notsufficiently exhibited. Even if the component (A) is used in an amountexceeding 60% by weight, further effect based on the increase in amountis not expected. Rather, the viscosity of the adhesive tends to beexcessively lowered in addition to lowering of the softness of theadhesive, since the content of the component (B) decreases.

The amount of the photopolymerization initiator (C) is preferably from 1to 20% by weight, more preferably from 3 to 15% by weight, based on thetotal amount of the polymerizable components (component (A) andcomponent (B)). If the amount of the component (C) is less than 1% byweight, the surface of the cured product tends to become sticky owing todecrease in curability. If the amount is more than 20% by weight, thereis a tendency that the component (C) is easy to be recrystallized orredeposited or the moisture resistance is deteriorated because thecomponent (C) is easy to migrate into the information recording layerafter the curing.

The adhesive of the present invention containing the components (A) to(C) as mentioned above may be incorporated with at least one of analicyclic mono(meth)acrylate (component (D)), in order to reduce theviscosity of the adhesive or to improve the adhesion property with asubstrate made by polycarbonate or the like and the moisture resistance,and a bisphenol di(meth)acrylate (component (E)), in order to impart arigidity to the adhesive so as to raise the strength of the adhesivelayer in addition to imparting a some degree of moisture resistance.

In case of incorporating the adhesive containing the components (A) to(C) (hereinafter also referred to as “adhesive (I)”) with an additionalpolymerizable component such as component (D) or component (E), it ispreferable to supplement the component (C) so that the proportion of thephotopolymerization initiator (C) is from 1 to 20% by weight, especiallyfrom 3 to 15% by weight, based on the total amount of the polymerizablecomponents.

The component (D) has one (meth)acryloyl group, and are monomers havinga group for example:

Since it has a relatively low molecular weight and a relatively lowviscosity, the viscosity of the adhesive can be reduced. In addition,since it is an alicyclic compound, the adhesion property can be furtherincreased as compared with mono(meth)acrylate compounds having anaromatic or linear aliphatic group.

Examples of the alicyclic mono(meth)acrylate (D) are, for instance,tetrahydrofurfuryl mono(meth)acrylate, cyclohexyl (meth)acrylate,dicyclopentenyl(meth)acrylate, isobornyl(meth)acrylate,norbornyl(meth)acrylate, dicyclopentanyl(meth)acrylate, adamantyl(meth)acrylate, tricyclodecane(meth)acrylate,hexacyclo-[6,6,1,1^(3.6),1^(10.13),0^(2.7),0^(9.14)]heptadecyl-4-acrylate,12-methylhexacyclo-[6,6,1,1^(3.6),1^(10.13),0^(2.7),0^(9.14)]heptadecyl-4-acrylate,octacyclo-[8,8,0,1^(2.9),1^(4.7),1^(11.18),1^(13.16),0^(3.8),0^(12.17)]docosyl-5-acrylate,15-methyl-octacyclo[8,8,0,1^(2.9),1^(4.7),1^(11.18),1^(13.16),0^(3.8),0^(12.17)]docosyl-5-acrylate,tetra-cyclo[4,4,0,1^(2.5),1^(7.10)]dodecyl-3-acrylate,2,7-dimethyltetracyclo-[4,4,0,1^(2.5),1^(7.10)]dodecyl-3-acrylate,9-stearyltetracyclo-[4,4,0,1^(2.5),1^(7.10)]dodecyl-3-acrylate, and thelike. Of these, tetrahydrofurfuryl mono(meth)acrylate, cyclohexyl(meth)acrylate and dicyclopentenyl (meth)acryalte are preferred from theviewpoints that the dilution property is good and the cured productsbecome tough.

In case of adding the component (D) to the adhesive (I), it ispreferable that the amount of the component (D) is from 5 to 100 partsby weight, especially from 10 to 80 parts by weight, based on 100 partsby weight of the total of the components (A) and (B). If the amount ofthe component (D) is too small, the effect produced by the use of thecomponent (D) is not sufficiently obtained. If the amount is too large,there is a tendency that the heat resistance is lowered and theviscosity becomes too low.

Examples of the bisphenol di(meth)acrylate (E) are, for instance,di(meth)acrylates produced from bisphenol compounds such as bisphenol A,bisphenol AD, bisphenol F and bisphenol S (component (E-1)),di(meth)acrylates produced from known bisphenol type epoxy resinsderived from bisphenol compounds (component (E-2)), and oligomers ofthese monomeric compounds.

Examples of the commercially available component (E-1) are, forinstance, NEW FRONTIEW BPE-4 and NEW FRONTIEW BPE-10 (which are productsof DAI-ICHI KOGYO SEIYAKU CO., LTD.), EBECRYL 150 (which is a product ofDAICEL-UCB COMPANY LTD.), and KALYARAD R-551 (which is a product ofNIPPON KAYAKU CO., LTD.). Examples of the commercially availablecomponent (E-2) are, for instance, SP1509, SP1506 and SP1507 (which areproducts of Showa Kobunshi Kabushiki Kaisha), and EBECRYL 3700, EBECRYL3720 and EBECRYL 600 (which are products of DAICEL-UCB COMPANY LTD.).These may be used alone or in admixture thereof. Of these, EBECRYL 150and EBECRYL 3700 are preferred from the viewpoints of high moistureresistance and good heat stability.

In case of adding the component (E) to the adhesive (I), it ispreferable that the amount of the component (E) is from 5 to 95 parts byweight, especially from 5 to 50 parts by weight, based on 100 parts byweight of the total of the components (A) and (B). If the amount of thecomponent (D) is too small, the effect produced by the use of thecomponent (D) is not sufficiently obtained. If the amount is too large,there is a tendency that the viscosity becomes high, so the adhesivelayer becomes hard and the adhesive strength is lowered. In particular,when the component (E-2) is used, the adhesive is apt to have a highviscosity and, therefore, it is preferable to select the amount withinthe range of 5 to 30 parts by weight. Also, in order to prevent the diskperformances (e.g. falling ball impact resistance) from lowering, it ispreferable that the amount of the component (E) does not exceed theamount of the component (B).

When both the component (D) and the component (E) are incorporated intothe adhesive (I), there can be obtained a more excellent adhesive havingwell balanced workability (viscosity), mechanical properties, heatresistance and the like based on the effects of both components. In thiscase, the amount of the component (D) is from 5 to 100 parts by weight,especially 10 to 80 parts by weight, per 100 parts by weight of thetotal of the components (A) and (B), and the amount of the component (E)is from 5 to 95 parts by weight, especially 10 to 80 parts by weight,per 100 parts by weight of the total of the components (A) and (B),provided that it is preferable that the total amount of the components(D) and (E) is from 10 to 150 parts by weight, especially from 10 to 130parts by weight, per 100 parts by weight of the total of the components(A) and (B), since the water resistance and the adhesion property areeasy to be lowered if the total amount of the components (D) and (E)becomes too large with respect to the total amount of the components (A)and (B). Also, it is preferable that the amount of the component (E)does not exceed the amount of the component (B).

Preferable examples of the formulations (wherein the total amount of thecomponents (A) and (B) is 100 parts by weight) for the adhesive of thepresent invention are, for instance, in case of the adhesive (I),compositions containing (A) 20 to 50 parts by weight ofN-vinylpyrrolidone, (B) 80 to 50 parts by weight of KRM7595, and (C) 1to 20% by weight of 1-hydroxycyclohexyl phenyl ketone based on the totalamount of the components (A) and (B). In case of the adhesiveincorporated with the alicyclic mono(meth)acrylate (D), preferableexamples are, for instance, compositions containing (A) 5 to 60 parts byweight of N-vinylpyrrolidone, (B) 40 to 95 parts by weight of KRM7595,(D) 10 to 80 parts by weight of tetrahydrofurfuryl acrylate based on 100parts by weight of the total of the components (A) and (B), and (C) 1 to20% by weight of 1-hydroxycyclohexyl phenyl ketone based on the totalamount of the components (A), (B) and (D). In case of the adhesiveincorporated with the bisphenol di(meth)acrylate (E), preferableexamples are, for instance, compositions containing (A) 5 to 60 parts byweight of N-vinylpyrrolidone, (B) 40 to 95 parts by weight of KRM7595,(E) 5 to 50 parts by weight of EBECRYL 150 based on 100 parts by weightof the total of the components (A) and (B), and (C) 1 to 20% by weightof 1-hydroxycyclohexyl phenyl ketone based on the total amount of thecomponents (A), (B) and (E). Also, in case of the adhesive incorporatedwith the components (D) and (E), preferable examples are, for instance,compositions containing (A) 5 to 60 parts by weight ofN-vinylpyrrolidone, (B) 40 to 95 parts by weight of KRM7595, and basedon 100 parts by weight of the total of the components (A) and (B), (D)10 to 80 parts by weight of tetrahydrofurfuryl acrylate and (E) 10 to 80parts by weight of EBECRYL 150, and (C) 1 to 20% by weight of1-hydroxycyclohexyl phenyl ketone based on the total amount of thecomponents (A), (B), (D) and (E).

The adhesive of the present invention may further contain other knownpolymerizable monomers such as monofunctional (meth)acryates,polyfunctional (meth)acrylates, vinyl acetate and (meth)acrylic acid, orother known polymerizable oligomers, so long as the features of thepresent invention are not impaired. In this case, it is preferable tosupplement the photopolymerization initiator (C) so that the proportionof the component (C) is from 1 to 20% by weight based on the total ofthe polymerizable components.

Examples of the monofunctional (meth)acrylate are, for instance,(meth)acrylates having, as a substituent group, a group such as methyl,ethyl, propyl, butyl, amyl, 2-ethylhexyl, octyl, nonyl, dodecyl,hexadecyl, octadecyl, cyclohexyl, benzyl, methoxyethyl, butoxyethyl,phenoxyethyl, nonylphenoxyethyl, glycidyl, 2-hydroxyethyl,2-hydroxypropyl, 3-chloro-2-hydroxypropyl, dimethylaminoethyl,diethylaminoethyl, nonylphenoxyethyltetrahydrofurfuryl,caprolactone-modified tetrahydrofurfuryl or dicyclopentenyloxyethyl.

Examples of the polyfunctional (meth)acrylate are, for instance, adi(meth)acrylate of 1,3-butylene glycol, 1,4-butanediol,1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, neopentylglycol, 1,8-octanediol, 1,9-nonanediol, tricylodecanedimethanol,ethylene glycol, polyethylene glycol, propylene glycol or polypropyleneglycol, a di(meth)acrlate of tris(2-hydroxyethyl)isocyanurate, adi(meth)acrylate of a diol obtained by addition of 4 moles or more ofethylene oxide or propylene oxide to 1 mole of neopenyl glycol, adi(meth)acrylate or tri(meth)acrylate of a triol obtained by addition of3 moles or more of ethylene oxide or propylene oxide to 1 mole oftrimethylolpropane, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate,trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate,dipentaerythritol poly(meth)acrylate, a poly(meth)acrylate ofcaprolactone-modified tris[(meth)acryloyloxyethyl]isocyanurate oralkyl-modified dipentaerythritol, a poly(meth)acrylate ofcaprolactone-modified dipentaerythritol, hydroxypivalic neopentylglycol, ethylene oxide-modified phosphoryl (meth)acrylate, ethyleneoxide-modified alkylphosphoryl (meth)acryalte, and the like.

Examples of the polymerizable oligomer are oligomers of polyester(meth)acrylates other than the above-mentioned, polyether(meth)acrylate, epoxy (meth)acrylate or urethane (meth)acryalte, and thelike.

The polymerizable monomer is effective for reducing the viscosity of theadhesive. The polymerizable oligomer is effective for adjusting themodulus of elasticity of the adhesive layer. The monofunctional(meth)acrylate is effective for increasing the adhesion property of theadhesive. The polyfunctional (meth)acrylate is effective for improvingthe strength of the adhesive.

These components should be used so as not to impair the features of thepresent invention, that is, in an amount of at most 20% by weight,especially at most 10% by weight, based on the total amount of allpolymerizable compounds.

The adhesive of the present invention may contain, as occasion demands,various assistants and additives, e.g., silane coupling agent,polymerization inhibitor, levelling agent, surface lubricating agent,defoaming agent, light stabilizer, antioxidant, antistatic agent, fillerand organic solvent.

Respective adhesives according to the present invention can be obtainedby uniformly mixing the components. Uniform mixing can be achievedgenerally by stirring the components in a mixer or a kneader at atemperature of 40 to 80° C. under a reduced pressure (5 to 30 mmHg).

The adhesive of the present invention is specifically explained by meansof examples, wherein all parts and % are by weight unless otherwisenoted. It is to be understood that the present invention is not limitedto these examples.

Examples 1 to 7 and Comparative Examples 1 to 7

The components shown in Tables 1 and 2 were mixed according to therecipes shown in Tables 1 and 2 at 60° under a reduced pressure of 10mmHg for 0.5 hour by a mixer to give a uniform ultraviolet curableadhesive.

The water resistance of the obtained adhesive and the corrosionresistance of the information recording layer were evaluated by thefollowing methods. The results are shown in Tables 1 and 2.

(Water resistance)

1. Moisture staying property

An optical disk having a diameter of 120 mm and a thickness of 0.6 mmwas radially cut into 16 equal parts and washed with isopropyl alcohol.As shown in FIG. 1, the defatted disk part 1 was coated with theprepared ultraviolet curable adhesive so that the thickness of theadhesive layer became about 10 μm, and another defatted disk part 2 wassuperposed thereon with the information recording layers facing eachother so as not to introduce bubbles to the oblique line portion in FIG.1. The adhesive was cured by UV irradiation from a metal halide lamp(made by JAPAN STORAGE BATTERY CO., LTD.) under a condition of 500mJ/cm². After the curing, the test specimens were boiled for 1 hourunder atmospheric pressure, and they were taken out and allowed to standat room temperature (25° C.). With respect to each test specimen,generation of marks of water drops (polka dots) was observed every hourup to the eighth hour. The evaluation was made according to thefollowing criteria.

◯: No mark of water drops was observed.

Δ: Several marks of water drops were observed several hours later.

X: An enormous number of marks of water drops were observed on theentire adhered surfaces.

2. Adhesion property

The test specimen subjected to the test of moisture staying property waspeeled by fixing the portion a shown in FIG. 1 with one hand and theportions b and c with another hand and pulling in the verticaldirection. The evaluation was made according to the following criteria.

◯: The specimen cannot be peeled.

Δ: The specimen cannot be peeled, but can be peeled if scored by acutter along the adhesive layer.

X: The specimen can be easily peeled by hand.

(Corrosion resistance of information recording layer)

The adhesive was applied to each deposition film of a polycarbonatesubstrate having an Al deposition film and a polycarbonate substratehaving an SiC deposition film, and a test specimen as shown in FIG. 1was prepared in the same manner as above. Using a polycarbonatesubstrate having an Al deposition film and a polycarbonate substratehaving an Si deposition film, a test specimen was also prepared in thesame manner. The prepared test specimens were allowed to stand in athermo-hygrostat at 80° C. and a humidity of 95% for 168 hours, and thestate of each deposition film was observed. The evaluation was madeaccording to the following criteria.

◯: There is no change in hue of the surface and there is no pinhole ineach deposition film.

Δ: There is no pinhole, but there is a change in hue of the surface.

X: Change in hue of the surface and pinholes are observed in thedeposition film.

Abbreviations of respective components shown in Tables 1 and 2 are asfollows:

Component (A)

N-VP: N-vinylpyrrolidone

Component (B)

KRM7595: Urethane acrylate oligomer prepared using a caprolactone-basedpolyol as a raw material, trade mark KRM7595 (made by DAICEL-UCB COMPANYLTD.)

Component (C)

D1173: 2-Hydroxy-2-methyl-1-phenyl ketone, trade mark Dalocure 1173(made by Ciba Specialty Chemicals K.K. Japan) (C-1 component)

IC184: 1-Hydroxycyclohexyl phenyl ketone, trade mark Irgacure 184 (madeby Ciba Specialty Chemicals K.K. Japan) (C-1 component)

IC907: 2-Methyl-1-[4-(methylthio)phenyl]-2-morpholino-propane-1-one(made by Ciba Specialty Chemicals Co., Ltd.) (C-2 component)

L-TPO: 2,4,6-Trimethylbenzoyldiphenylphosphine oxide, trade markLucitin-TPO (made by BASF Japan Ltd.) (C-2 component)

Component (D)

THF-A: Tetrahydrofurfuryl acrylate

Component (E)

EB150: Diacrylate prepared using bisphenol as a raw material, trade markEBECRYL 150 (made by DAICEL-UCB COMPANY LTD.) (E-1 component)

EB3700: Diacrylate prepared using a bisphenol type epoxy resin as a rawmaterial, trade mark EBECRYL 3700 (made by DAICEL-UCB COMPANY LTD.) (E-2component)

Other components

AC-M: Morpholine acrylate

EB270: Urethane acrylate oligomer prepared using a polyether polyol as araw material, trade mark EBECRYL 270 (made by DAICEL-UCB COMPANY LTD.)

PhEA: Phenoxyethyl acrylate

TABLE 1 Example No. 1 2 3 4 5 6 7 Composition (part) N-VP (A) 50 50 2525 35 10 10 KRM7595 (B) 50 50 50 50 50 40 40 D1173 (C-1) 5 — — — — — —IC184 (C-1) — 5 5 5 5 5 5 IC907 (C-2) — — — — — — 1 THF-A (D) — — 25 — —30 30 EB150 (E-1) — — — 25 — 20 20 EB3700 (E-2) — — — — 15 — —Evaluation results Water resistance 1. Moisture staying property ◯ ◯ ◯ ◯◯ ◯ ◯ 2. Adhesion property ◯ ◯ ◯ ◯ ◯ ◯ ◯ Corrosion resistance ofInformation recording layer 1. SiC/Al disk ◯ ◯ ◯ ◯ ◯ ◯ ◯ 2. Si/Al disk ◯◯ ◯ ◯ ◯ ◯ ◯

TABLE 2 Comparative Example No. 1 2 3 4 5 6 7 Composition (part) N-VP(A) 50 50 — — — 10 — AC-M — — 10 — — — 20 KRM7595 (B) — 50 40 50 — — —EB270 50 — — — 50 40 30 L-TPO (C-2) — 5 — — 5 — — IC184 (C-1) 5 — 5 5 —5 5 THF-A (D) — — 30 — — 30 30 EB150 (E-1) — — 20 — — 20 20 PhEA — — —50 50 — — Evaluation results Water resistance 1. Moisture stayingproperty X ◯ Δ X X X X 2. Adhesion property X X X ◯ X ◯ X Corrosionresistance of Information recording layer 1. SiC/Al disk Δ X ◯ Δ X Δ Δ2. Si/Al disk Δ X X Δ X Δ Δ

INDUSTRIAL APPLICABILITY

Optical disks laminated using the ultraviolet curable adhesive of thepresent invention exhibit an excellent moisture resistance without anyadverse influence of moisture on the information recording layerthereof. In particular, an excellent moisture resistance is exhibited bythe use of the ultraviolet curable adhesive of the present inventioneven in the case that the optical disks have a thick translucent film(information recording layer).

I claim:
 1. An ultraviolet-curable adhesive for bonding optical disks,which comprises: (A) N-vinylpyrrolidone, (B) a urethane (meth)aerylateoligomer derived from a caprolactone-based polyol, an alicyclicdiisocyanate and (meth)acrylic acid, and (C) a photopolymerizationinitiator containing (C-1) a photopolymerization initiator composed ofonly carbon atoms, hydrogen atoms and oxygen atoms without containing anitrogen atom, a sulfur atom and a phosphorus atom and (C-2) aphotopolymerization initiator containing a nitrogen atom and/or a sulfuratom in a (C-1)/(C-2) ratio of 70/30 to 100/0 by weight, wherein saidultraviolet-curable adhesive bonds optical disks.
 2. The adhesive ofclaim 1, wherein only the component (C-1) is used as saidphotopolymerization initiator (C).
 3. The adhesive of claim 1, whereinthe content of the component (A) is from 5 to 60% by weight based on thetotal amount of the components (A) and (B).
 4. The adhesive of claim 2,wherein the content of the component (A) is from 5 to 60% by weightbased on the total amount of the components (A) and (B).
 5. The adhesiveof claim 1, 2 or 3, which further contains (D) 5 to 100 parts by weightof an alicyclic mono(meth)acrylate per 100 parts by weight of the totalof the components (A) and (B).
 6. The adhesive of claim 1, 2 or 3, whichfurther contains (E) 5 to 95 parts by weight of a bisphenol typedi(meth)acrylate per 100 parts by weight of the total of the components(A) and (B), provided that the amount of the component (E) does notexceed the amount of the component (B).
 7. The adhesive of claim 1, 2 or3, which further contains (D) 5 to 100 parts by weight of an alicyclicmono(meth)acrylate and (E) 5 to 95 parts by weight of a bisphenol typedi(meth)acrylate per 100 parts by weight of the total of the components(A) and (B), provided that the amount of the component (E) does notexceed the amount of the component (B).
 8. The adhesive of claim 1,wherein said photopolymerization initiator (C) contains said initiator(C-1) and said initiator (C-2) in a (C-1)/(C-2) ratio of 70/30 to 99/1by weight.
 9. The adhesive of claim 8, wherein said initiator (C-1) isat least one member selected from the group consisting of1-hydroxy-cyclohexyl phenyl ketone,2-hydroxy-2-methyl-1-phenylpropanone,2,2-dimethoxy-2-phenylacetophenone, benzophenonc, benzoin isobutylether, benzoin n-butyl ether, 2,2-dimethoxy-1,2-diphenylethane-1-one,methylphenyl glyoxylate and benzyl dimethyl ketal, and said initiator(C-2) is at least one member selected from the group consisting ofMichler's ketone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone,2-isopropylthioxanthone, 2-chloro-thioxanthone,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,3,6-bis(2-methyl-2-morpholinopropanonyl)-9-butyl-carbazole,isoacryl-4-(dimethylamino)benzoate, ethyl-4-(dimethylamino)benzoate,(4-(4-methylphenylthio)phenyl)-phenylmethanone and2-methyl-1-(4-(methyl-thio)phenyl)-2-morpholino-propane-1-one.
 10. Anultraviolet-curable adhesive for bonding optical disks, which comprises:(A) N-vinylpyrrolidone, (B) a urethane (meth)acrylate oligomer derivedfrom a caprolactone-based polyol, an alicyclic diisocyanate and(meth)acrylic acid, and (C) aphotopolymerization initiator containing(C-1) a photopolymerization initiator composed of only carbon atomshydrogen atoms and oxygen atoms without containing a nitrogen atom, asulfur atom and a phosphorus atom and (C-2) a photopolymerizationinitiator containing a nitrogen atom and/or a sulfur atom in a(C-1)/(C-2) ratio of 70/30 to 99/1 by weight.
 11. The adhesive of claim10, wherein the content of the component (A) is from 5 to 60% by weightbased on the total amount of the components (A) and (B).
 12. Theadhesive of claim 10, which further contains (D) 5 to 100 parts byweight of an alicyclic mono(meth)acrylate per 100 parts by weight of thetotal of the components (A) and (B).
 13. The adhesive of claim 10, whichfurther contains (E) 5 to 95 parts by weight of a bisphenol typedi(meth)acrylate per 100 parts by weight of the total of the components(A) and (B), provided that the amount of the component (E) does notexceed the amount of the component (B).
 14. An ultraviolet-curableadhesive for bonding optical disks, which comprises: (A)N-vinylpyrrolidone, the amount of which is from 5 to 60% by weight basedon the total amount of the components (A) and (B), (B) a urethane(meth)acrylate oligomer derived from a caprolactone-based polyol, analicyclic diisocyanate and (meth)acrylic acid, (C) a photopolymerizationinitiator containing (C-1) a photopolymerization initiator composed ofonly carbon atoms, hydrogen atoms and oxygen atoms without containing anitrogen atom, a sulfur atom and a phosphorus atom and (C-2) aphotopolymerization initiator containing a nitrogen atom and/or a sulfuratom in a (C-1)/(C-2) ratio of 70/30 to 100/0 by weight, (D) 5 to 100parts by weight of an alicyclic mono(meth)acrylate per 100 parts byweight of the total of the components (A) and (B), and (E) 5 to 95 partsby weight of a bisphenol type di(meth)acrylate per 100 parts by weightof the total of the components (A) and (B), provided that the amount ofthe component (E) does not exceed the amount of the component (B). 15.The adhesive of claim 14, wherein said photopolymerization initiator (C)contains said initiator (C-1) and said initiator (C-2) in a (C-1)/(C-2)ratio of 70/30 to 99/1 by weight.